Metal halide arc discharge lamps are frequently employed in commercial usage because of their high luminous efficacy and long life. A typical metal halide arc discharge lamp includes a quartz or fused silica arc tube that is hermetically sealed within a borosilicate glass lamp envelope. The arc tube, itself hermetically sealed, has tungsten electrodes attached into opposite ends and contains a fill material including mercury, metal halide additives and a rare gas to facilitate starting. In some cases, particularly in high wattage lamps, the lamp envelope is filled with nitrogen or another inert gas at less than atmospheric pressure. In other cases, particularly in low wattage lamps, the lamp envelope is evacuated.
It has been found desirable to provide metal halide arc discharge lamps with a shroud which comprises a generally cylindrical, light transmissive member, such as quartz, that is able to withstand high operating temperatures. The arc tube and the shroud are coaxially mounted within the lamp envelope with the arc tube located within the shroud. Preferably, the shroud is a tube that is open at both ends. In some cases, the shroud is open at one end and has a domed configuration on the other end. The shroud has several beneficial effects on lamp operation, which are known to those skilled in the art.
Sodium is an important constituent in most high intensity metal halide arc discharge lamps, usually in the form of sodium iodide or sodium bromide. Sodium is used to improve the efficacy and color rendering properties of metal halide lamps. It has long been recognized that arc tubes containing sodium lose sodium during discharge lamp operation. Sodium is lost by the movement, or migration, of sodium ions through the arc tube wall. The iodide originally present in a metal halide lamp as sodium iodide is freed by sodium loss, and the iodide combines with mercury in the arc tube to form mercury iodide. Mercury iodide leads to increased reignition voltages, thereby causing starting and lamp maintenance problems.
A number of designs have been proposed in the prior art for reducing sodium migration from metal halide arc discharge lamps. In U.S. Pat. No. 4,281,274 issued Jul. 28, 1981 to Bechard et al, a shroud is electrically biased with a DC voltage in order to repel positive sodium ions which have migrated through the wall of the arc tube. In a so called "frameless construction" disclosed in U.S. Pat. No. 3,424,935 issued Jan. 28, 1969 to Gungle et al, no frame members are located close to the arc tube. U.S. Pat. Nos. 4,620,125 issued Oct. 28, 1986 to Keeffe et al and 4,625,141 issued Nov. 25, 1986 to Keeffe et al disclose a metal halide arc discharge lamp wherein the metal straps used to support the shroud and the arc tube are electrically connected to an electrical lead of one polarity so that sodium loss from the arc tube is reduced. Other techniques for reducing sodium loss from arc discharge lamps are disclosed by Keeffe et al in Journal of Illumination Engineering Society, Summer 1988, pages 39-43; U.S. Pat. No. 4,963,790 issued Oct. 16, 1990 to White et al; Japanese Patent No. 60-40138 published Jul. 30, 1976 and U.S. Pat. No. 4,843,266 issued Jun. 27, 1989 to Santo et al. U.S. Pat. No. 5,023,505, issued Jun. 11, 1991 to Ratliff et al, discloses an arc discharge lamp wherein a support is attached to a lamp stem using a stem clip. U.S. Pat. No. 5,136,204 issued Aug. 4, 1992, discloses a metal halide arc discharge lamp structure including a frame comprising one or two support rods, and upper and lower clips for retaining the shroud and the arc tube. The clips, which are welded to the support rod, prevent both axial and lateral movement of the shroud. The frame is attached to the base end of the lamp by a strap which encircles the lamp stem. Although the lamps disclosed in U.S. Pat. No. 5,156,204 are mechanically strong and are able to survive shipping and handling without significant breakage, these lamps have been found to have a shorter operating life than is known to be achievable. The lamps exhibit changes over life which are indicative of sodium loss.
A further disadvantage of the lamps disclosed in U.S. Pat. No. 5,136,204 is that the neck region of the lamp envelope sometimes contacts the strap which secures the frame to the lamp stem during heat sealing of the lamp envelope to the lamp stem. When this occurs, the lamp envelope must be replaced, thereby increasing manufacturing costs. In addition, the procedure for attaching the strap to the lamp stem during lamp manufacturing is inconvenient and relatively costly.
A metal halide arc discharge lamp wherein the arc tube, shroud and frame are supported by a plurality of resilient springs is disclosed in U.S. Pat. No. 5,065,069, issued Nov. 12, 1991. In the disclosed arc discharge lamp, mechanical connections to the lamp stem are eliminated. Although the disclosed lamp uses bulb spacers at the lower end of the frame, additional angled springs are required to prevent axial movement of the frame relative to the lamp envelope. The additional springs add to the cost and complexity of the lamp. In addition, springs contacting the main portion of the lamp envelope wall are likely to damage any phosphor coating that may be present.
It is a general object of the present invention to provide improved arc discharge lamps.
It is another object of the present invention to provide arc discharge lamps wherein sodium migration from the arc tube is suppressed.
It is a further object of the present invention to provide arc discharge lamps which are capable of withstanding mechanical shock and vibration.
It is still another object of the present invention to provide arc discharge lamps which have long operating lives.
It is another object of the present invention to provide arc discharge lamps wherein leakage current between a support frame and electrical inleads is effectively eliminated.
It is a further object of the present invention to provide arc discharge lamps which are simple in construction, easy to manufacture and low in cost.